1. Field of the Invention
The invention is directed to a method for the production and playback of a topographic image of a subject from a radiological tomogram, particularly in the imaging of X-rays on a digital pickup system for the generation of tomograms, particularly panorama tomograms.
2. Description of the Prior Art
In X-ray exposures, in digital X-ray exposures as well, the beam pathxe2x80x94proceeding from a radiation source, the focusxe2x80x94passes through the transirradiated subject wherein the focal plane lies, for example the upper or lower jaw regions, before the rays impinge the sensor plane wherein the subject is imaged. The respective distances between the focal plane and the focus, and between the focal plane and the sensor or reception plane, usually are derived from the dimensions of the subject to be transirradiated and from the pickup conditions.
The beam path from the radiation source through the focal plane into the plane receiving the radiation follows the laws of the radiation theorem. Emanating from the radiation source, the ray beam proceeds through the focal plane that is located in the subject to be imaged. The subject has its true size, in the scale of 1:1, in the focal plane. Proceeding from the subject plane, the ray beam continues to expandxe2x80x94now influenced in intensity and contrast by the subjectxe2x80x94and as a result the subject of the focal plane is enlarged in the vertical and horizontal directions dependent on the distance of the focal plane from the sensor or reception plane and, thus, is distorted. The subject, for example individual teeth or entire sections of the upper or lower jaw, are imaged enlarged in the reception plane by a factor that lies between 1.05 and 1.24 as viewed in the vertical direction. Without correction, a distorted presentation of the subject on the imaging plane arises in X-ray exposures, even in digital X-ray exposures.
The vertical and horizontal distortion of individual regions of the X-ray image must be taken into consideration in the diagnosis of such an X-ray image and the interpretation of special characteristics based on the X-ray image. The distorted regions of the X-ray image must be visually interpolated in order to achieve true-to-scale conditions on the X-ray image. This can lead to errors in the interpretation, particularly arising from the spatial allocation that is frequently difficult to estimate.
Heretofore, the above factors have been addressed by determining the actual size conditions of the region to be diagnosed from the film or from the printout given digital X-ray exposures with the assistance of back-calculations, templates or from the radiation theorem given a known magnification factor between the subject and the reception plane.
German OS 196 19 925 discloses that the position and spatial orientation of the examination subject and the changes thereof during the implementation of the exposure given panorama X-ray devices for producing tomograms be acquired by measurement with a position detector. Correction signals are then formed from the signals of the position detector in order to allocate the correct image contents to the subject details from the desired slice to be sharply imaged before the implementation of the tomographic blurring effect and to follow up the position of the desired slice with respect to the patient movement. This size correction only serves to assist in the interpretation of the image contents acquired in the individual frames with topical precision for generating a tomosynthetic image reconstruction. The complicated method of tomosynthetic image reconstruction, however, is not required for the production of panorama tomograms.
In accordance with the present invention, in a method and x-ray apparatus for making and reproducing a radiological tomogram of a subject, the subject is irradiated with radiation emanating from a focus disposed at a known distance from a focal plane, situated at the subject. The focal plane is disposed at a known distance from a radiation receiver, which is disposed in the image plane, and which contains a number of picture elements. In a digital radiological tomogram, either the slice position with the aforementioned distances is allocated to respective picture elements, or a correction factor, based on the slice position and the aforementioned distances, is allocated to the respective picture elements, from which a corrected image is obtained with the subject appearing with a corrected size corresponding to the size of the subject in the focal plane.
The inventive method has a number of advantages. The diagnostic content of digital X-ray exposures can be considerably improved with the method since geometrical conditions can be matched to one another better and uncertainty factors that occur given back-calculations and due to estimating are precluded. By means of the correction of the magnification factor to, for example, the true-to-scale size of 1:1 prevailing in the subject plane, an additive overall image can be produced in a number of slice positions and be played back on the picture screen in many types of presentation.
In a further embodiment of the inventive method, a digital imaging in the image plane can be registered with a sensor having pixels generating picture elements arranged in rows and columns. The buildup of the digital imaging ensues by column-by-column readout of the pixels from the sensor during the rotation of a focus around a center. During the rotation of the focus around the center along the focal curve, the coordinates of a defined slice position are allocated to the picture elements of the registered subject. This allocation of the subject and the slice position to the correction values, and or the correction values themselves, are preferably stored; so that this information can then be accessed again.
A correction of the magnification factor then ensues such that each picture element for a prescribed slice position is recalculated in height relative to the height that is present in the focal plane, i.e., for example, of the upper or lower jaw to be imaged.
In a further embodiment of the invention, the picture elements can also be recalculated in width for each prescribed slice position.
The picture elements recalculated relative to the height of the subject plane, i.e. of the focal plane, can then be imaged in the image plane corrected in this way, and, for example, can be visually presented true-to-scale without distortion in the scale 1:1 on a monitor of a picture screen. This true-to-scale imaging without distortion obtained by recalculation of the picture elements can subsequently be further-processed in many ways.
Advantageously, an overall image is calculated from all recalculated slice positions, this overall image, in particular, being able to be implemented such that the subject is imaged in the corrected exposure in the same size which it has in the focal plane.
The method is especially advantageous when picture elements having different spacings are generated and/or present in a slice position of the radiological tomogram, so that different depths of field can be produced within a slice position and the different imaging scales can thereby be taken into consideration. The fundamental principle for producing tomograms with different depth information or, respectively, depth of field is disclosed by German OS 197 33 338 which corresponds to U.S. Pat. No. 6,049,584, the teachings of which are incorporated herein by reference.
The inventive correction method can be especially advantageously employed in an X-ray device for registering digital X-ray exposures, wherein a focus is movable along a focal curve around a center, a sensor lies in the image plane, and the image size of the picture elements lying in the image plane for prescribed slice positions is converted onto a corrected image size of the subjects in the focal curve, for which purpose means are provided wherein the correction factors belonging to the picture elements of a slice position are stored. As a result, it is possible to implement the inventive correction dependent on the slice position.
Again with reference to German OS 197 33 338, an X-ray apparatus is designed such that, for subsequent determination of the sharp slice and/or of the depth of field, the sensor is composed of a number of narrow zones that can be read out, to generate tomographic image information with high depth of field and predetermined slice position. This image information is individually read out and calculated for an image of slight depth of field in an image processing unit, whereby the distance of this image from the focal plane can be determined by varying the topical offset of the individual tomographic images with the assistance of an input device that influences the image processing.